Boiler systems wherein water tanks are heated either by interior means or by exterior means are well known. Furthermore, it is known that a hot water boiler may have two separate hot water outlets, one for domestic hot water and the other for a heating system in the building. Examples of such devices are shown in Meyers U.S. Pat. No. 2,937,625 and in Pompei et al U.S. Pat. No. 4,222,350. Both of these systems however show complicated units wherein water or liquid is circulated from a tank and the heating occurs when the water is not in the tank. Such systems are generally expensive and complex.